How can I make particles remain stationary on a surface?
There are two basic ways to approach this problem through particles and collision objects:

First is not very suitable for animations, since the leaves freeze at the moment they hit the collision object. You basically check the "Die on Hit" and "Died" check-boxes in the Particle settings.



Second is more suitable for animations since the leaves move a bit even after hitting the ground and come to a stop smoothly. Here you DON'T check the "Die on Hit" and "Died" check-boxes but you edit the settings for the collision object itself. Namely the Stickiness and Friction Factor values.



Either way, the big problem is keeping leaves not going through the ground thanks to rotation of the particle - the collisions don't take into account the shape of the leaves. I would need more context to choose a solution for this. One solution would probably be to let the leaves rotate only around Z axis (thus not going through XY planes) which would tamper with the animation of their fall though and work only on absolutely flat XY planes.